CROSS-REFERENCE TO RELATED APPLICATIONS
This application is related to concurrently filed U.S. patent applications Ser. Nos. 08/173,396 (Attorney's Docket No. FI9-93-004) and 08/173,383 (Attorney's Docket No. FI9-93-005), both of which are assigned to the assignee of the present invention which are hereby fully incorporated by reference herein.
1. Field of the Invention
The present invention generally relates to high resolution photolithography and, more particularly, to improved fabrication methods for high performance masks for use in making photolithographic exposures.
2. Description of the Prior Art
The formation of fine patterns of conductors and other portions of circuit elements is an indispensable part of the fabrication of integrated circuits and other electronic devices, such as multi-layer modular circuits which may contain many such integrated circuits and other devices which are connected by conductive patterns on lamina thereof. Photolithography techniques are well-known and highly developed for the production of such patterns.
In general, photolithography involves the application of a photosensitive resist material to a surface of a lamina, substrate or partially formed integrated circuit and the exposure of a portion of the photosensitive resist material in accordance with a desired pattern. The pattern is then developed by selective removal of either the exposed or unexposed portion of the resist (depending on whether the resist material is a positive or a negative resist) allowing material to be selectively removed or deposited in accordance with the remaining pattern of resist material.
The exposure of the photoresist material is often accomplished by the projection of light or other radiation (e.g. at ultra-violet and shorter wavelengths) through a mask since a mask permits a high degree of accuracy, repeatability and convenience as compared to direct writing of the pattern. The quality of the mask therefore determines and limits the quality of the developed pattern of resist material. While very high quality mask patterns have been developed, however, some optical effects have further limited resist exposure quality.
Specifically, due to the wave-like nature of light and other forms of radiation suitable for photolithography processes, diffraction and other interference effects occur at the edges of opaque areas of the mask and may cause a dimensional change (or produce ghost patterns) in the exposed pattern since the opaque regions of the mask cannot be placed directly in contact with the photo resist during exposure. In practice, it is customary to project the image using an optical system of substantial length in order to achieve a reduction of the size of the pattern at the resist surface relative to the size of the mask, often by a factor of four or more. These effects therefore cause some spreading of the exposed image or even the exposure of additional regions of the photoresist corresponding to lobes of energy radiating at an angle to the plane of the mask from an aperture therein, depending on the separation of the opaque regions of the mask from the photoresist (e.g. the distance over which the pattern image is projected).
While this effect is generally dimensionally small, recent increases in integration density of integrated circuits has pushed minimum feature sizes of patterns into regimes where the effect has become significant and often critical to high manufacturing yields. Therefore, to improve exposure patterns, a so-called rim phase-shift mask has been developed in order to limit image spreading in exposure of features of closed shape. A similar phase-shift mask formation known as a Levenson-type shifter is used for exposure of periodically repeated patterns, such as arrays of parallel conductors.
Essentially, such rim phase-shift masks provide an altered optical path length through the mask at the edge or rim of the opaque pattern formed in the mask. A Levenson-type phase shift structure provides a similar effect with differing path lengths between opaque regions which enhances contrast of repeated patterns, such as parallel lines. In a rim phase-shift mask, the difference in path length usually is designed to provide a 180.degree. relative (to 0.degree.) phase shift of the radiation at the wavelength at which the exposure is made. Levenson-type and similar shifters usually are designed to provide several amounts of relative phase shift such as 90.degree. and 180.degree.. This relative phase shift causes an interference effect which slightly narrows the exposure pattern at the photoresist surface relative to the size of an aperture formed in the mask and reduces the intensity of radiation beyond the edges of the aperture (e.g. the energy in the side lobes) such that any exposure which occurs beyond the dimensions of the mask aperture is insufficient to be developed.
While rim phase-shift masks have been made and effectively used, the fabrication of the masks has been difficult and expensive due to the need to form extremely small regions having differing optical lengths at the edges of opaque regions. That is, either patterning must be done within the mask pattern or the opaque regions of the mask must be recessed from the regions of differing optical path length. Additionally, the amount of phase shift is critical to the performance of the mask in order to attenuate side-lobes of radiation due to diffraction. While a phase shift of 1800 is generally sufficient for a rim phase shift mask which is used to form closed shapes, other phase shift structures, such as Levenson-type phase shifters which are used to form arrays of parallel conductors employ a sequence phase shift regions having relative phase shifts of 0.degree., 90.degree., 180.degree. and 90.degree. between parallel opaque mask areas.
In either case, the phase shift is often obtained by etching a substrate to corresponding depths to alter the optical path length through the substrate or other transparent mask material. However, preferred methods of anisotropic etching, such as reactive ion etching (RIE) proceed at rates which are subject to wide variation which is not readily controlled. A tolerance of 15% in blind etching depth is generally regarded as the highest accuracy which can be consistently attained even under the most stringent of process conditions.
To obtain higher accuracy for high-performance phase-shift structures, an etch stop layer can be used. However, an etch stop layer must necessarily be of a different material than the transparent regions of the mask and a close match of refractive indices between the etch stop material and the substrate or other transparent material of the mask is usually impossible. This mismatch of refractive indices causes a partially reflective interface which reduces the transparency of the mask, reduces exposure efficiency and complicates mask design, particularly where several etch stop layers must be employed to obtain several different phase shifts because of different degrees of transparency which are thus produced. Also, when an etch stop region remains embedded in the mask, two interfaces are produced by each etch stop layer so embedded. Differing transparencies of areas for each amount of phase shift desired is thus usually unavoidable and further complicates mask design. While amounts of transmitted and reflected light can be equalized to some degree by careful control of the thicknesses of the etch stop and transparent material layers in the mask, such control is difficult and expensive as well as complicating of the mask design.
Spin-on glass (SOG) is sometimes also used for the phase shift layer or layers. However, SOG is considered less desirable since thermal treatment is required to remove the organic component of the SOG layer (as well as to relieve some of the stresses induced by the spin on process which alter refractive index of the material). Also, the glass material, itself, is not presently suitable for use in the deep ultraviolet region and shorter wavelengths of the electromagnetic spectrum due to increased absorption of these wavelengths relative to other materials.
Recently, a liquid phase deposition process has been introduced which has successfully deposited silicon oxide on a substrate surface at low temperature. Specifically, silicon oxide has been deposited on a surface at 35.degree. C. using a solution of hydrofluorosilicic acid (H.sub.2 SiF.sub.6) and boric acid (H.sub.3 BO.sub.3) which is supersaturated with silica (SiO.sub.2), as disclosed in "A New Process for Silica Coating" by Hirotsuga Nagayama et al., published in the Journal of the Electrochemical Society, August, 1988, pp.2013-2016, which is fully incorporated by reference. In summary, the solubility of silica is said to decrease with increasing temperature and a saturated solution can be made supersaturated by increasing temperature. However supersaturation is achieved, the excess silica is then deposited on a surface immersed in the supersaturated solution while the concentration of resulting hydrofluoric acid is minimized by reaction with the boric acid.
While the possibility of low temperature oxide deposition by liquid phase silica deposition is attractive for forming insulators between interconnect layers, heat treatment at 500.degree. C. or higher (essentially an annealing process to form a more orderly silica network of increased density) has been found to improve dielectric quality and increase density of the oxide with some increase in refractive index. However, these temperatures are not feasible where metal deposits (e.g. for conductors) or critical distributions of dopants or impurities are present since temperatures above 350.degree. C. will damage or destroy the integrity of metal conductors and cause accelerated levels of diffusion of dopant or impurity materials. Some selectivity of deposition of silica from the liquid phase using the process disclosed in the above incorporated article has also been reported: "A New Interlayer Formation Technology of Completely Planarized Multilevel Interconnection by Using LPN" by T. Homma et al., published in the 1990 Symposium of VLSI Technology suggests that a difference in wettability between the resist and the intended deposition surface can improve selectivity of deposition. However, the profiles of deposits shown in micrographic images published therein exhibit relatively large variations in thickness, particularly at edges of deposits, which would render them largely useless for phase-shift mask features or other optical applications where optical path length through the deposit was of importance.